Monday, August 4, 2014

Interaction of Appliances

One of my readers (thanks Andreas) commented about how controlling the appliances could help me.  For example, not letting the compressors of the refrigerator and freezer go on at the same time could lower my peak demand.  I answered that I was a bit leery about that because it could lead to spoiled food.  But, I decided an exercise in looking at it wouldn't take too much time and maybe it would tell me if such a thing would save me money.

For those of you just stumbling upon this site, I have demand billing from my wonderful power company.  There's a long discussion of this here <link>, but suffice it to say that from noon to 7PM I carefully control the usage of power around the house to reduce the power bill.  A single mistake that lasts for 15 minutes during this period could cost me a LOT of money.  Enough to buy several little computers to control things around the house.

So, I looked at adding the usages and creating a new graph to show the total use from my two freezers and refrigerator over a period of time, but had a brainstorm along the way.  I could simply use HighCharts ability to stack graphs.  It worked, and now I have a better understanding of my usage that I want to illustrate for people out there that are considering (or doing) the same kind of thing I do.

First though, a recap of the appliances and what their power usage profile is.  Here's my refrigerator graph as an area chart:

This appliance runs for short bursts to cool down to around 38-40 degrees.  It gets the most activity, but is pretty efficient.  Now, my freezer:

This thing runs a lot.  It has a good compressor that doesn't use much power, but it runs a lot more than I expected.  Still, it doesn't actually chew up much power.  And last, the garage freezer:

This thing runs about half the time during the summer because the garage is hot.  I don't cool the garage and it's on the south side.  

The little spikes in the charts are an artifact of the way I graph the data.  Yes, I could have hunted down the problem and fixed it, but I wanted to study the data, not spend a couple of hours chasing down a bug and fixing it.  So, just ignore the spikes, they only mean I'm lazy.

And finally, the stacked composite graph:

Spikes aside, this shows that my peak usage for all three devices is less than 500 watts.  Sure, it would be more energy efficient to stop one to allow a different one to run, but it wouldn't change my peak usage much.  The way they (the power company) company calculate this is to take a moving average of 15 minutes over the entire peak period for a month and then they bill me for the highest period.  So, any 500 watt period will cost me for the entire month.  I once messed up and used a couple of kW, so I had the freedom to really eat the power for the rest of the month.  Interesting result of peak demand billing, I wonder if they realize it.

At any rate, it doesn't look like I have to worry much about the combination of appliances getting my usage out of control, but it could be useful to keep this in mind if I add an appliance, or need to modify my lifestyle in the future.  Most people don't want to put up with their A/C units being shut off during the hottest part of the day, or their pool motor not running in the afternoon or early evening when they want to use it.  Fortunately, when relatives visit me, they understand my OCD about keeping appliances shut off, and just giggle about me when they get home, so I don't have to worry about them.

See folk, I actually take your suggestions seriously and even act on them from time to time.  It may take months, but I eventually get there.  Now I still have to think about getting a temperature sensor inside the freezer (thanks badhairday).